In cellular radio communication systems, one radio base station typically supports several cells radio communication with users in these cells. FIG. 1a is an illustration of a typical cell plan, with each of the radio base stations 110 serving three cells A, B, and C, in separate 120-degree sectors. It should be understood that a cellular radio network comprises a high plurality of cells A, B, C, and a corresponding plurality of radio base stations 110 that serve user equipment 150 with communication services in these cells A, B, C. FIG. 1a is a simplification of a total cellular radio system, as it illustrates only two radio base stations 110a, 110b, one of which serves the three cells, A, B and C. When building networks of this type, it is often difficult to find and get access to sites for the radio base station and antennas. Accordingly, a cell plan as is illustrated in FIG. 1a has the advantage of enabling geographical coverage with a restricted number of base station 110 sites. A further advantage by having one radio base station 110a, serving plural cells A, B, and C is that the base station's physical hardware and software resources can be shared among the cells that are served. For example, when traffic is non-uniformly distributed among the cells A, B, C served by the radio base station 110a, the computing resources available in the radio base station 110a are distributed such that more is used for taking care of traffic in the more heavily loaded one or ones of the three cells.
FIG. 2a illustrates an example architecture for a radio base station 110, having a radio equipment (RE) 112 for each of the three cells A, B, C, and a common radio equipment controller (REC) 114. The REs 112 and the REC 114 are connected via a standardized interface named the Common Public Radio Interface (CPRI). The REC communicates baseband signals and control information over the CPRI and handles all the baseband signaling processing of signals received by the REs and signals to be transmitted by the REs, and further controls the communications in the cells. The CPRI allows for a flexible construction and building of a radio base station.
A basic approach to handling increasing traffic load in a cellular system is to introduce more cells. However, the difficulty in finding sites is limiting operators' ability to increase the number of cells. One way to increase capacity with a fixed set of base station sites is the introduction of distributed RE, i.e., RE that share the same REC on one site. In this approach, antennas and REs are spread to support radio communication in spaced apart, or in more or less overlapping cells A, B, C, as illustrated in FIG. 1b, while the control of the communications is made by the REC. An example of an architecture for this type of radio base station is illustrated in FIG. 2b, with the REC 114 connected to the spaced apart REs 112 via the CPRI.
In many cellular systems, the physical radio resources on the air interface are shared among a plurality of active users based on their immediate need for communications. One such system is the Long Term Evolution (LTE) wireless system as specified by 3GPP. The radio interface for LTE is commonly named the Evolved Universal Terrestrial Radio Access (E-UTRA), which radio interface will be used in the discussion that follows as an example. E-UTRA uses orthogonal frequency division multiplexing (OFDM) in the downlink (DL) from an eNB (3GPP terminology for a base station in LTE systems) to user equipments (UEs, 3GPP terminology for mobile terminals or end stations) or terminals, and discrete Fourier transform (DFT)-spread OFDM in the uplink (UL) from a UE to an eNB.
The basic LTE downlink physical radio resource may be seen as a time-frequency grid as illustrated in FIG. 3, where each resource element, i.e. square in the grid, represents one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 4a. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, also called Physical Resource Blocks (PRB), where a resource block corresponds to one timeslot of 0.5 ms in the time domain and twelve contiguous subcarriers in the frequency domain, as illustrated in FIG. 4b. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Scheduling is the process of assigning resources on the physical radio resource to the active users in a cell based on their respective need for communication. The UEs 150 in a cell at which data is buffered for DL transmission in the radio base station 110 are candidates for being assigned DL transmission resources. Similarly, among the UEs 150 in a cell that have requested UL resources, some UEs are selected for being granted UL transmission resources. Scheduling is based on the need for communication by the UEs, as is typically defined by UE specific scheduling weights that are provided to the scheduling process.
The UE transmissions weights are based on the amount of data buffered waiting for transmission, the type of service the UE is involved in, and associated QoS attributes such as low latency requirements, priority, etc. For the DL communication direction, this information is available in the radio base station, while for the uplink scheduling weights are produced based on information received from the UEs.
The scheduling process generally assigns resources for a period of 1 ms, which constitutes a transmission time interval (TTI) of 1 ms. The scheduling process is therefore repeated for each TTI, and the TTI period is the same as a sub-frame, i.e. 1 ms. Resource blocks scheduled to one UE in the UL direction must be allocated contiguously in the frequency domain, while there is no such restriction for the DL direction.
For each TTI, the radio base station 110 informs the UEs 150 of the resources assigned in the DL by DL assignments sent to scheduled UEs. Similarly, resources assigned in the UL direction are assigned by UL grants sent to the assigned UE. These assignments and grants are transmitted on a common DL control channel, the PDCCH (physical downlink control channel), which is carried by the first 1, 2, 3, or 4 OFDM symbol(s) in each subframe and which spans over the whole OFDM carrier bandwidth. A UE that has decoded the control information carried by a PDCCH knows which resource elements in the subframe contain data aimed for the UE. The length of the control region, in which the PDCCH is located, can vary from subframe to subframe.
Several characteristics of the PDCCH should be noted. First, the PDCCH is a costly resource. The OFDM symbol used in each subframe for the PDCCH steals resources from the physical downlink shared channel (PDSCH) that typically carries downlink data traffic. The more resources used for the PDCCH in terms of OFDM symbols, the fewer are left for carrying downlink data. For example, if three of the fourteen ODFM symbols in a frame are occupied by the PDCCH, only 79% of the subframe's capacity is left for carrying data while 21% is spent for carrying control information. Second, the PDCCH is shared by all UEs in the cell, which compete for both UL and DL transmission resources. Third, the PDCCH is cell specific.
It should also be noted that in a radio base station where signal processing resources are shared among plural cells for processing the signals of the UE in those cells, there is a limitation on the number of UEs that are possible to process.
Accordingly, for efficient use of the PDCCH and of the signaling processing capacity, care must be taken when selecting the UEs that get access to the two types of resources.